The invention relates to a process for the catalytic exhaust gas aftertreatment of engine combustion emissions by an NOx storage catalyst which can be regenerated by feeding hydrogen to the exhaust gas upstream of the NOx storage catalyst.
In the case of lean-operated internal-combustion engines, particularly in the case of direct-injection Otto engines, Diesel engines or hydrogen engines, which are operated at an overstoichiometric air-fuel ratio, it is necessary to reduce nitrogen oxides. In addition to continuously operating systems, intermittently operating NOx storage catalysts are used for this purpose which, in an overstoichiometric operation of the internal-combustion engine, intermediately store NOx, which can be released again in shorter understoichiometric operating phases. For example, a platinum-containing adsorber is used which oxidizes NO by oxygen first to NO2. In the subsequent storage step, the acidic NO2 then attacks a metal oxide and in the process forms the pertaining nitrate. In particular, alkali metals and alkaline earth metals (such as barium) can be used as storage material and, because of their basic characteristics, are capable of forming sufficiently stable nitrates in the given temperature range. When the supply of storage oxides is used up, the engine must be operated for a short time by a rich mixture in order to regenerate the storage device. In this case, the reducing agents present in the rich mixture break up the nitrates and NO is created as well as the original metal oxide.
In the implementation of this concept, particularly the change from the lean to the rich operation required for the regeneration of the NOx adsorbent is disadvantageous. In addition to the very high control expenditures, there are also the risk of HC and CO slip as well as a particle/soot emission.
Remedies were provided by regenerating the NOx storage catalyst by adding hydrogen to the untreated exhaust gas upstream of the NOx storage catalyst. German Patent Document DE 199 39 807 A1 describes such a process and a system for the exhaust gas aftertreatment particularly for lean-mix engines, such as direct-injection Diesel engines and gasoline engines, as well as maintaining the operability of NOx storage catalysts in gasoline and diesel engines and particle filters in diesel engines. For the regeneration of the NOx storage catalyst, hydrogen is intermittently added to the untreated exhaust gas approximately once per minute in order to cause a reduction of the nitrogen oxides. According to German Patent Document DE 199 39 807 A1, a high-expenditure on-board hydrolysis unit is provided for obtaining the hydrogen, which hydrolysis unit, in addition to the actual electrolyzer, comprises a water reservoir, a metering device, a hydrogen reservoir for the intermediate storage and a piping system.
This unit requires the costs of a separate hydrogen process that includes the process-related devices required exclusively for the regeneration of the NOx storage catalyst. The hydrogen used for the regeneration of the NOx storage catalyst has to be made available only for the purpose of regenerating the catalyst and, if required, has to be intermediately stored in a hydrogen reservoir. The water reservoir has to be refilled regularly.
It is therefore an object of the invention to provide a process which overcomes the above-mentioned disadvantages.
According to the invention, this object is achieved by using a process which includes the withdrawal of the hydrogen used in a secondary process for the regeneration of the NOx storage catalyst from a primary hydrogen process.
Within the scope of the invention, the term “hydrogen” also applies to a hydrogen-rich synthesis gas.
In the case of a motor vehicle, whose internal-combustion engine can be operated with hydrogen, it is very expedient, if a partial quantity is extracted from the hydrogen flow fed to the internal-combustion engine and fed into the exhaust gas flow upstream of the NOx storage catalyst. Depending on the further design of the exhaust gas aftertreatment device, the feeding of the hydrogen to the exhaust gas flow takes place discretely or continuously.
In the case of a motor vehicle whose internal combustion engine is operated with hydrocarbon fuels and at which motor vehicle the hydrogen containing synthetic gas is produced in a primary hydrogen process by a reformer, especially in order to operate a fuel cell system, it is very advantageous, if a partial quantity of this hydrogen-rich reformer gas is fed into the exhaust gas flow upstream of the NOx storage catalyst.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.